1. Field of the Invention
This invention relates to instruments for measuring a length, and more particularly to an instrument for measuring a length of a workpiece to be measured from a value of displacement in the axial direction of a spindle.
2. Description of the Prior Art
Heretofore, there have been developed instruments for measuring a length of various types, including a very common one, a so-called thread micrometer, in which internal threads are formed with precision on an inner sleeve solidly secured to a main frame, external threads of a spindle also formed with precision are threadably coupled to the internal threads, and a thimble integrally affixed to this spindle is rotated to thereby measure a length of the workpiece. The instrument for measuring a length of the type described can offer such advantages that the internal mechanism including the threads, having a substantially hermetically closed construction, is high in dust excluding properties, the spindle is not freely rotated due to a self-locking action of the threads even if a measurer releases his hand from the thimble, so that the workpiece can be reliably maintained in a clamped state.
On the other hand, the instrument for measuring a length of the type described presents such disadvantages that, since the pitches of the threads are as minute as about 0.5 mm in general, the value of bite of the threads, i.e., the threadably coupled position of the threads is varied and hence the measuring accuracy is not stabilized depending on the value of a thimble operating force when zero point is set or the workpiece to be measured is clamped, thus requiring a high skill level in measuring operation. Additionally, since the pitches of the threads are so minute as described above, the spindle cannot be transferred at high speed, and hence, particularly, the working efficiency of repeated measuring operation is low, and moreover, there is such a complicated measuring work requiring for reading graduations and verniers formed on an outer sleeve coupled to the inner sleeve and the thimble. Further, no operation at high speed can be effected as described above, and moreover, the spindle is directly threaded, and hence, the spindle is rotated in measuring operation, whereby, at the time of measuring a material high in flexibility, such as a soft plastics plate, wrinkles and the like are caused to the workpiece made of such a material as described above. Hence, the instrument for measuring a length of the type described is not suitable for measuring the workpieces made of the material as described above, and further, is not suitable for one-hand operation because the thimble moves in the axial direction of the spindle while rotating in measuring operation. Furthermore, such a disadvantage is presented that the instrument for measuring a length is high in production cost because finishing work with high accuracy is required for threading and graduation forming.
Further, there has been known a thread type linear instrument for measuring a length, in which the spindle is transferred linearly while not rotating with the screw-feed being still adopted. In this linear type instrument for measuring a length, an intermediate cylindrical member is rotatably and axially non-slidably mounted onto the outer periphery of the spindle and precision threads formed on the outer periphery of this intermediate cylindrical member are threadably coupled to threads formed on an inner sleeve, whereby, although improvements are made in its linearity, other disadvantages are not obviated, and moreover, such a new problem in production has been presented that the intermediate cylindrical member must be assembled and adjusted between the spindle and the inner sleeve in a manner to be parallel and coaxiall therewith.
For the purpose of attaining high speed operation, there has been proposed a linear type instrument for measuring a length, in which a pinion of a rotary knob rotatably supported on a main frame is meshed with a rack formed on a spindle, and this rotary knob is rotated so as to transfer the spindle at high speed.
The linear type instrument for measuring a length as described above should necessarily have the advantages of the high speed operation and nonrotatableness of the spindle. However, if one hand is released when the workpiece to be measured is clamped, then the spindle automatically moves in a direction of being separated from the workpiece, and hence, a necessity is created for a separately provided position holding device for preventing the spindle from being relieved, i.e., a so-called locking device. In connection with the above, a constant pressure device must be provided because it is difficult to lock the spindle in predetermined position, and moreover, releasing means for the above-described locking device should have been added.